This invention relates to radioactive light sources and in particular to radioactive light sources that function to emit significant amounts of electromagnetic radiation without the aid of any externally applied energy source.
Light sources powered by radioactive sources have been known for many years. One of the early applications of such a source involved the use of luminescent coatings for watch dials and instrument panels in which the coating consisted of a radium compound admixed with a ZnS:Cu phosphor. See, for example, H. E. Leverenz, "An Introduction to the Luminescence of Solids", John Wiley and Sons, 1950. Aside from their health hazards, such luminescent sources are of limited utility because of their very low brightness which, for example, may be about 0.02 fL. For applications where larger area light sources of higher brightness are required, other means for generating the luminescence with the aid of a radioactive material has been studied. The most successful of these has involved the use of a radioactive gas, such as krypton-85 or tritium, whose beta ray emission excites a cathodo-luminescent phosphor exposed to the gas. During recent years such applications have mostly employed tritium because of the desirable energy range of its beta rays and the fact that it does not emit any penetrating radiation which would present a health hazard. Examples are U.S. Pat. No. 3,478,209 to Feuer relating to self luminous tritium light sources and U.S. Pat. No. 4,213,052 to Caffarella et al relating to radioactive light sources comprising a phosphor coated tube filled with tritium gas and such sources supplemented internally with radiation-to-voltage transducers.
In the usual arrangement, the light source consists of a glass tube whose inner surface is first coated with a layer of inorganic phosphor powder. Following this, tritium gas is admitted into the tube, which is then sealed off, providing a self contained light source requiring no external source of power to generate the light. Although the half-life of the tritium is sufficiently long for most practical purposes, about 12.3 years, a major limitation is the low brightness of such tubes. Unlike a suitable cathodoluminescent phosphor, such as ZnS, of optimized thickness and a tritium pressure of the order of 1 atmosphere, for example, the surface brightness produced is only about 1 fL. Although adequate for restricted applications, such as airport markers for night viewing, the brightness achieved is far below the level desired for many other applications.
To a large degree, the above limitation in brightness results from the fact that at useful pressures of tritium only beta rays (energetic electrons) originating from tritium atoms relatively close to the phosphor surface are effective in producing the light, since beta rays produced by tritium atoms more distant from the phosphor are blocked by self absorption of other tritium atoms. In the case of air, for example, at atmospheric pressure, the average range of the beta rays is only about 0.5 mm. In this connection, see the article of A. Korin et al entitled, "Parameters Affecting the Intensity of Light Sources Powered by Tritium", Nuclear Instruments and Methods, Vol. 130, pp 231-237, 1975. Although, to some degree, a higher brightness can be obtained by increasing the tritium pressure to increase the density of data rays in the vicinity of the phosphor, this is counteracted by the fact that beta ray from a correspondingly thinner layer of tritium can reach the phosphor. In principle, an alternative method for increasing the brightness would be to provide a stack of multiple phosphor layers on separate substrates separated a small distance from each other within a tritium filled chamber so that the light generated at each layer can be viewed through the layers behind it. Unfortunately, because of light absorption and light scattering in the phosphor layers, only a limited gain in brightness can be achieved in such a scheme.
It is an object of this invention to make more effective use of the beta emission from tritium by mixing with it a luminescent gas which can be excited throughout its volume by the emitted beta rays. Since such a gas mixture can be totally transparent, light generated at all depths will reach the surface of the tube, producing a substantial brightness from a tube containing a given volume of tritium.